1.Field of the Invention
The present invention relates to a turbo compound engine that is capable of recovering the energy of exhaust gas as an expansion work of the turbine and utilizing the recovered energy as rotative power of a drive shaft such as a crank shaft. In particular, it concerns a turbo compound engine that is capable of generating an optimum and minimum engine braking force in accordance with the running condition of the engine.
2. Background Art
Generally, supercharged engines have advantages in comparison with non-supercharged engines of larger displacements: (1) better fuel consumption rate, (2) equal or better output performance, and (3) lighter weight and compactness.
A turbo compound engine is an engine in which the above advantages are further augmented. In the turbo compound engine, the energy of exhaust gas from the engine is recovered by the turbocharger as its supercharging work, and the remaining energy of the exhaust gas from the turbocharger is recovered by the power turbine as its adiabatic expansion work. This construction brings about a general improvement in the power output performance, fuel consumption rate, and gain of the engine. Further, the turbo compound engine's overall performance or its serviceability can be further ameliorated by increasing the expansion ratios of the turbocharger and the power turbine so as to raise its supercharging pressure.
In this case, however, it remains a problem to secure an adequate braking effort (for example, by means of exhaust braking) to counterbalance the increased power output of the engine. In other words, as a relative engine braking force suffers a decrease because of increased turbocharged pressure, a main brake (i.e., foot brake) should be manipulated in order to offset the relative decrease of entire braking force.
Obtaining a sufficiently large engine brake force is important not only for the maneuverability and safety of the vehicle (engine brake force of approximately more than 60% of the rated output power is required), but also for taking better advantage of the advantages of the turbo compound engine. Thereupon, the present assignee has proposed a "Turbo Compound Engine" disclosed in Japanese Patent Application No. 61-308776 which is included herein by reference.
In this proposal, as shown in FIG. 6 of the accompanying drawings, a power turbine a and a crankshaft b are connected to each other by a gear train d, which has an electromagnetic clutch c, so as to transmit the rotation of the crankshaft to the power turbine a. Said electromagnetic clutch is connected upon exhaust braking. The gear ratio of the gear train d is set smaller than that of the gear train e that transmits the rotative force of the power turbine to the crankshaft b when the exhaust brake is not operated.
The exhaust brake is generally manipulated even when the engine is running beyond the rated revolution speed. In this case, there is a substantial possibility that the power turbine may go into an overrun condition. Therefore, in the above proposal, during exhaust braking, the electromagnetic clutch is actuated so as to transmit the rotation of the crankshaft to the power turbine, rotating the power turbine in the reverse sense. In this manner, compared with the gear ratio of the gear train which transmits the rotation from the power turbine to the crankshaft when the exhaust brake is not operated, the gear ratio of the other gear train which transmits the rotation from the crank gear to the power turbine is set small, so that the overrun of the power turbine upon exhaust braking is prevented.
Also according to the above idea, the power turbine functions as a compressor, and the inlet line of the power turbine is throttled to a certain degree in order to acquire a braking force corresponding to the degree of throttling of the inlet line as depicted in FIG. 7. In this case, however, the throttling of the inlet line is fixed. Hence, a characteristic or an advantage that the braking force varies with the revolution speed of the power turbine is not utilized effectively. FIG. 8 shows that the braking force increases with the revolution speed of the power turbine. Therefore, it is desirable to obtain an appropriate braking effort in accordance with the revolution speed of the power turbine.
In addition, according to the construction of the above proposal, it is possible to obtain an energy absorption force as braking force of more than one-third of the engine horsepower at an optimum throttling. However, if the whole of the acquired braking force works at the same time when the vehicle is running, the following difficulties will arise:
(1) the vehicle skids momentarily; PA1 (2) an anti-driving force upon skidding exerts an extremely large load on the driving system of the vehicle; and PA1 (3) an abnormal abrasion of tires, brake pads and shoes occurs.